Kaboom!!!

Monday 25th August 2008. 2hr

Tidied up after priming. Assembled both aft spars. Dissassembled left leading edge. Marked leading edge ribs for position. Stripped blue vinyl from rivet lines of left leading edge skin.

It was dark by the time I finished last night, so everything was just thrown back into the shop. This evening had to start with a tidying session to make the shop habitable (and workeable) again.

I had just clecoed the parts of the two aft spars together and set up for the first rivets when I noticed that I had no pressure in the hose, even though the compressor had run normally when I switched it on and had cut out after about the right amount of time. On opening the compressor cabinet, I could see that the regulator knob was missing. I found it on the floor of the cabinet with some other parts. The pressure adjustment knob is plastic and fits to a threaded plastic moulding that screws into the top of the regulator. The knob varies the compression in a spring beneath it, which pushes through a plastic flap sitting on top of a valve between the top outlet chamber and a filter chamber below. Everything above this valve had blown off.

Nothing seemed damaged so I tried just screwing it back. No luck. It just blew again more or less immediately. Fortunately, the compressor was bought new and is just entering the last three weeks of its one year guarantee. I took the complete regulator off to return to the supplier tomorrow.

So I had to find some other jobs to make myself useful for the rest of the evening. Getting the blue vinyl off the rivet lines of the skins is always a good fallback to fill in time. As that involved disassembling the left leading edge, I went on to mark the ribs for position, using the same scheme of punch-marks beside holes that I had used for the main ribs.

Primed and ready

Sunday 24th August 2008. 9hr 15m

Etched main wing ribs and aft spars and doublers and primed them all.

What a long day. Fortunately, it was the first genuine summer's day in Dublin since June. The rest of the summer has been a complete wash-out, for the second year in a row. It makes you wonder whether there will be enough flying weather to justify owning a plane.

I switched back and forwards between the etching parts and priming them. The sunshine was very helpful in drying the parts quickly. On the other hand, when refilling the paint hopper on the gun, the paint was extremely slow to pass through the 125micron strainer I was using. This may be an indication that the paint was too thick but I did not want to over-thin it. On the emp, I felt the paint was too thin by the time it got to the recommended viscosity as measured with a Dupont viscosity cup. this time, I decided to just 'wing' it.

Each of the two coats of primer is itself a two step process, as parts need to be turned to get at the other side. With the spray booth, I can hang quite a lot of parts and avoid having to turn them. Fortunately, the primer dries in about 10-15 minutes (noticeably longer at lower temperatures, as I noticed when evening drew on and the light faded)

As the day wore on, the last parts got etched so I was sitting around waiting for parts to dry so that I could shoot more primer. By that time, I was glad of a chance to just sit in the shop and read ahead through the manual.

Ready to Prime

Saturday 23rd August 2008. 5hr 30min

Completed tank attach platenuts on left spar. Set up for priming. Marked remaining parts "right"/"left". Cleaned ribs for left wing and aft spar parts. Etched and 1st coat primed all aft spar doublers.


My Mum & Dad came for a visit last night so I got a night off. Coming back with a fresh mind to those three platenuts on the aft face of the spar near the root, I eventually got them done without much additional drama. I was probably just tired on Wednesday by the time I got to that job.


So, back to priming. This is not a job I enjoy. I envy builders who just take out a rattle can and shoot, like it was something that barely needed mentioning. This Stewarts primer/sealer system requires two steps (cleaning and etching) before shooting anything. The shooting itself, like all spray painting, involves significant set-up and climbing into a sweaty overall and mask for the job itself. The last time I did it was in winter, so I set up a spray booth, which took up half the workshop and left the remainder very cramped. This time, I am hoping for good weather and I intend to do the spraying outside.

From my experience with the emp, I learned that the cleaner will take off sharpie markings so it is important to mark each part in a way that will still be seen after the primer goes on. As each part on the aft spar is unique, it is only necessary to distinguish between those for the left wing and those for the right. I decided to use a pattern of dots made with the centre punch for this. I chose to use ":.." for left and "..:" for right. This can still be confused depending on which way up you read them so I adopted the convention of always putting them on the aft face of the part , near the bottom.

I had not forgotten how tedious the job of preparing the parts is and it fully lived up to expectations. Cleaning the parts thoroughly with the Stewarts cleaning compound (diluted as per the instructions) and a piece of red Scotchbrite pad is a lot like washing the pots after dinner, which I am also not fond of. Never having been caught short of cash in a restaurant, I have never had to spend most of a day at the sink, but this was a good simulation.


After the scrubbing step, I rinsed off each part and left it to drain (dry if possible). Next it has to be painted with Ekoetch (diluted with an equal volume of water). This has to be left on for about three minutes before thoroughly rinsing the part and letting it dry. At that stage it is finally ready for the primer. Getting a process going to move all the parts through these steps certainly keeps you on your toes.

Spray set up

27th December 5 hr

Setting up for priming

I got a few hours away from the Christmas festivities today to get ready for the priming operation.

The access to the booth proved a problem because the velcro is so agressive and because,when you pull the sides apart, you are shortening the height of the walls so the floor has to come up. This made getting in and out, particularly in full painting regalia while holding parts, very awkward. Because of this, I taped up the old corner entrance and cut a full-sized roll-up door in one of the side walls. The 'door' was cut from spare Powerclad material as it had to overlap the opening. I re-used the velcor (expensive stuff) tape to fix it to the opening at three points down each side and one across the bottom. This works quite well. When I take the booth down I might consider using a long zip to close the 'door'; just as you would find in a tent. I could then open or close it single handed in one movement.

I found some useful advice on setting up a spray gun in the Stewarts procedures manual and supplemented this with some more information that I found on-line. There was quite a bit of trial and error involved so I taped some newspaper to the inside of the booth and filled the gun with water. Using something with a colour in it would make this trial and error process a lot easier but would create more mess. I didn't want to waste any primer so I just made do. In the end, I got what I thought was a nice narrow vertical oval shaped spray pattern and the liquid seemed to be coming out of the nozzel at a reasonably good rate.

Primer

Most people think that aluminium, from which the RV-9 is built, does not corrode. Unfortunately, this is not quite the case. It is true that pure aluminium reacts with air to form a protective coating of aluminium oxide which is highly resistant to corrosion. In this way, pure aluminium does indeed protect itself. However, the alloys of aluminium used in building aircraft do not have this feature. Some of the parts of the Vans range are coated at the mill with a thin layer of pure alluminium to restore this property. These are refered to as "alclad". The problem is that the coating is very thin and prone to scratching, leaving an entry point for corrosion. You can easily recognise alclad parts because they are covered in a light adhesive film of blue plastic to protect them from such scratches. At some stage however, this film has to come off. Then there is the large number of parts (including all pressed parts such as ribs and bulkheads and the lengths of stock sections used for stringers and longerons) that are not alclad in the first place.

Many US builders debate whether to prime their planes at all. Their warm dry climate may give them a choice but our atlantic climate gives us none at all. I don't know of any builder on this side of the pond who is not priming. I don't have hangerage for the plane sorted out yet and there is a distinct possiblity that it will be stored in the open for at least a period after it is finished. Consequently, it would not be sensible to leave corrosion protection to chance. That disposes of the first battleground of the 'primer wars', which is whether to prime at all or not.

The next question is where to prime. Well the counter question is why wouldn't you prime everything? The answer to that is three-fold; time, cost and weight. These are the main arguments quoted by those who do not favour priming. For me, it is a question of balancing these factors against the need for additional corrosion protection.

One option is to prime everything that is not alclad. Even on an alclad part, the cut edges are not protected. In addition, you are drilling holes through the metal for rivets and sometimes cutting a part such as a stiffner to length, thereby creating more edges. So you would have to prime these areas anyway. Then there is the possibility that condensation will form inside the fuselage and will gather in crevices in the structure. This is where corrosion usually starts. These crevices will normally be at points where two metal parts are joined so it makes sense to prime at all such points. You could, for instance, prime along the narrow strips onto which stiffeners will be riveted. This would ensure that the rivet holes in the skin would be covered also. Even alclad parts riveted to the skin (such as stiffners) have edges and rivet holes where the aluminium alloy is exposed. As they are so small, it would not be worthwhile to prime only holes and edges so in practice the entire part must be done.

At this stage, you have to ask yourself, what areas of the aircraft structure would not be primed under such a regine and the answer is that it would amount to the central areas of the skin that are not close to an edge or close to another part. I started out this blog with the position that I would adopt this strategy but now I feel that leaviing these areas would be more trouble than it was worth.

The rivets themselves have always struck me as being a likely place for corrosion to get a foothold and also the one place you would not want this to happen. The only way of protecting these areas is to 'wet rivet', meaning that every rivet is dunked in primer before being set while still wet. I am interested in giving this a try. If I can develop a workable method, I will use it. Otherwise a compromise may be on the cards.

The next issue is which primer or primer system. There is a variety of products on the market in a variety of formats, such as rattle (aerosol) cans. Some of the primers claim to be self-etching, meaning that they roughen the surface at the microscopic level in order to improve the adhesion of the primer. This is achieved by adding a proportion of acid to the paint, which evaporates away with the rest of the 'carrier' solution in which the paint is delivered. These acids are not easy to incorporate into paint without some other major compromise and I don't believe in them. Etching is a vital step and I think it needs to be a separate step on its own.

One thing that almost all the primers seem to have in common is that they are toxic so, in practice, they are trying to kill you. Among the names you hear are "isocyanate" (cyanide) and "zinc chromate" (chromium). These are not substances you stir into your tea. That is why it is so vital to have really good breathing aparatus and even skin protection.

The automotive world is moving towards non-toxic water-based paints. So far, the only line of products I know of for the aviation world that is water-based are the products produced by a company called Stewart Systems. These have the important added benefits of being very low odor and easy to clean up. In fact, I recently saw a demonstration arranged by Aeropicardie, who are the European Agents for Stewart Systems at a SAAC meeting. Now the SAAC meetings are held in a restaurant. Can you imagine the potential for disaster if we were using toxic smelly chemicals in a restaurant of all places. As it happened, I can vouch for the fact that there was a stronger smell off the excellent cup-cakes that were served with our tea that evening and nobody got sick either.

This feature is particularly important to me, as I am building in such a small space and I don't have space for setting up an elaborate spray booth.

The Stewarts website also mentions another important factor in primer technology, which relates to permeability. The function of a primer is to provide a substrate which is a perfect environment for a finishing coat. One of the important elements of this environment is that is easy to adhere to. For this reason, most primers are actually porous, providing lots of microscopic gaps for the finishing coat to get down into and grip. Inside an aircraft structure, we do not use a finishing coat, for weight, time and cost reasons. So the main reason for using a primer (corrosion prevention) is actually negated by having a porous primer, without the protection of a finishing coat, that does not, in fact, prevent moisture from reaching the metal underneath. Two-pot epoxy primers are an exception here as they are fully impermeable. However they score especially badly on weight, time and cost. The Stewarts product is called a "Primer Sealer". It does completely seal the meetal and is designed to be left without a finishing coat. Together with its non-toxicity and very low odor, that makes it the perfect anti-corrosion system for a homebuilt plane.

So that's it. Decision made. The plane will be fully coated internally with the Stewarts system, incorporating Ekoclean cleaner, Ekoetch etching compound (obviously) and Primer Sealer; all water based, low odor and non-toxic - and friendly to the environment as well.